docs: def thermback(self):
          """Calculate thermal background count rate for ``self.obsmode``.
  
          Calculation uses
          :func:`~pysynphot.observationmode.ObservationMode.ThermalSpectrum`
          to extract thermal component source spectrum in
          ``photlam`` per square arcsec. Then this spectrum is
          integrated and multiplied by detector pixel scale
          and telescope collecting area to produce a count rate
          in count/s/pix. This unit is non-standard but used widely
          by STScI Exposure Time Calculator.
  
          .. note::
  
              Similar to IRAF STSDAS SYNPHOT ``thermback`` task.
  
          Returns
          -------
          ans : float
              Thermal background count rate.
  
          Raises
          ------
          NotImplementedError
              Bandpass has no thermal information in graph table.
  
          """
          #The obsmode.ThermalSpectrum method will raise an exception if there is
          #no thermal information, and that will just propagate up.
          sp=self.obsmode.ThermalSpectrum()
  
          #Thermback is always provided in this non-standard set of units.
          #This code was copied from etc.py.
          ans = sp.integrate() * (self.obsmode.pixscale**2 *
                                  self.obsmode.primary_area)
          return ans